Protesters stand in the middle of a busy intersection blocking traffic Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge officer Darren Wilson in the killing of 18-year-old Michael Brown. (AP Photo/Jeff Roberson)

Protesters stand in the middle of a busy intersection blocking traffic Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge officer Darren Wilson in the killing of 18-year-old Michael Brown. (AP Photo/Jeff Roberson)

Jihab Khayyam, of St. Louis, left, gets out of his car and honks his horn to show solidarity with demonstrators as they march through traffic protesting the August shooting of Michael Brown, Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge officer Darren Wilson in the killing of 18-year-old Brown. (AP Photo/David Goldman)

Brittany Ferrell, of St. Louis, chants "hands up don't shoot" while marching through the streets protesting the August shooting of Michael Brown, Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge officer Darren Wilson in the killing of 18-year-old Brown. (AP Photo/David Goldman)

Demonstrators block a busy intersection while marching through the streets to protest the August shooting of Michael Brown, Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge Officer Darren Wilson in the killing of 18-year-old Brown. (AP Photo/David Goldman)

Demonstrators block a busy intersection while marching through the streets to protest the August shooting of Michael Brown, Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge Officer Darren Wilson in the killing of 18-year-old Brown. (AP Photo/David Goldman)

Demonstrators block a busy intersection while marching through the streets to protest the August shooting of Michael Brown, Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge officer Darren Wilson in the killing of 18-year-old Brown. (AP Photo/David Goldman)

Police officers stand outside the Buzz Westfall Justice Center where a grand jury is convening to consider possible charges against the police officer who fatally shot Michael Brown in nearby Ferguson, Monday, Nov. 24, 2014, in Clayton, Mo. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge Officer Darren Wilson in the killing of 18-year-old Michael Brown. (AP Photo/David Goldman)

Grand jury reaches decision in Ferguson shooting

Protesters stand in the middle of a busy intersection blocking traffic Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge officer Darren Wilson in the killing of 18-year-old Michael Brown. (AP Photo/Jeff Roberson)

Protesters stand in the middle of a busy intersection blocking traffic Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge officer Darren Wilson in the killing of 18-year-old Michael Brown. (AP Photo/Jeff Roberson)

Jihab Khayyam, of St. Louis, left, gets out of his car and honks his horn to show solidarity with demonstrators as they march through traffic protesting the August shooting of Michael Brown, Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge officer Darren Wilson in the killing of 18-year-old Brown. (AP Photo/David Goldman)

Brittany Ferrell, of St. Louis, chants "hands up don't shoot" while marching through the streets protesting the August shooting of Michael Brown, Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge officer Darren Wilson in the killing of 18-year-old Brown. (AP Photo/David Goldman)

Demonstrators block a busy intersection while marching through the streets to protest the August shooting of Michael Brown, Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge Officer Darren Wilson in the killing of 18-year-old Brown. (AP Photo/David Goldman)

Demonstrators block a busy intersection while marching through the streets to protest the August shooting of Michael Brown, Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge Officer Darren Wilson in the killing of 18-year-old Brown. (AP Photo/David Goldman)

Demonstrators block a busy intersection while marching through the streets to protest the August shooting of Michael Brown, Sunday, Nov. 23, 2014, in St. Louis. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge officer Darren Wilson in the killing of 18-year-old Brown. (AP Photo/David Goldman)

Police officers stand outside the Buzz Westfall Justice Center where a grand jury is convening to consider possible charges against the police officer who fatally shot Michael Brown in nearby Ferguson, Monday, Nov. 24, 2014, in Clayton, Mo. Ferguson and the St. Louis region are on edge in anticipation of the announcement by a grand jury whether to criminally charge Officer Darren Wilson in the killing of 18-year-old Michael Brown. (AP Photo/David Goldman)

FERGUSON, Mo. (AP) — A grand jury has reached a decision about whether to indict a Ferguson police officer in the shooting death of Michael Brown, a spokesman for St. Louis County's top prosecutor said Monday.

In a brief email to reporters, spokesman Ed Magee said the decision would be announced at 8 p.m. at the downtown courthouse in the St. Louis County seat of Clayton. He offered no other details.

As the nation awaited the announcement, authorities quickly stepped up security around the courthouse. Barricades were erected around the building, and more than 20 Missouri state troopers were seen silently assembling with rifles, 3-foot batons, riot shields and other equipment. Some nearby businesses boarded up their windows, just as many shops have already done near the site of Brown's death in Ferguson.

Several school districts called off classes for Monday and Tuesday, extending the Thanksgiving break. More than 15 districts cancelled Monday evening activities. Washington University closed a satellite campus in Clayton.

The grand jury has been considering charges against Darren Wilson, the white suburban St. Louis officer who fatally shot the black 18-year-old after a confrontation in August.

The prosecutor's office has told Brown's relatives that the decision will be made public after 5 p.m., family attorney Benjamin Crump told The Associated Press.

The Aug. 9 shooting inflamed tensions in the predominantly black St. Louis suburb that is patrolled by an overwhelmingly white police force. As Brown's body lay for hours in the center of a residential street, an angry crowd of onlookers gathered. Rioting and looting occurred the following night, and police responded with armored vehicles and tear gas.

Missouri Gov. Jay Nixon traveled to St. Louis from the Capitol on Monday in advance of the grand jury announcement. He was to speak with St. Louis-area clergy on a conference call and then hold a news conference at the University of Missouri-St. Louis.

Pastors were planning a rally and prayer service later Monday at the West Side Missionary Baptist Church in St. Louis.

"There's a lot of hurt, a lot of brokenness. There's anger and frustration on every side," said the Rev. Ronald Bobo Sr., the church's pastor. "We need the hand of God to lead us and guide us."

The 12-person grand jury has been meeting in secret for months, hearing evidence from a wide variety of witnesses as it decides whether Wilson's should face charges that could range from involuntary manslaughter to murder. The grand jurors could decide not to charge Wilson at all.

At the lower end of the possible charges is second-degree involuntary manslaughter, which is defined as acting with criminal negligence to cause a death. It is punishable by up to four years in prison. The most serious charge, first-degree murder, can be used only when someone knowingly causes a death after deliberation and is punishable by either life in prison or lethal injection.